Tubular members such as drill pipe, tubing pipe and casing used in the oil and gas industry (herein referred to as "pipe sections") are joined at their ends by threaded connections. Power tongs and back-up tongs are used to make up (i.e. join) and break out (i.e. disconnect) these threaded connections. The back-up tong grips the lower stationary pipe section while the power tong grips the adjoining upper pipe section and rotates it to provide a fluid-tight seal during make up of the drill string, and to break out the threaded connection during disassembly of the drill string. Under-torquing a connection during make up may not provide the desired fluid-tight seal and adversely affect the integrity of the drill string, while over-torquing a connection may result in costly damages to the drill string, such as stripped threads on the pipe connection, and unnecessary operational delays. Hence, reliable torque readings during make up operations are increasingly important, particularly as equipment and manpower costs continue to rise.
The back-up tong is typically supported from and interconnected with the power tong by a support structure well known in the industry. Referring to the prior art assembly shown in FIG. 1, the frame structure normally includes a torque plate or leg 12 which extends downwardly from the power tong 10 and engages a rearwardly extending portion or heel 14 of the back-up tong 11. During make up operations the torque leg engages and bears against the back-up tong's heel to counteract the twisting forces of the power tong, although the torque leg is free to move vertically with the power tong relative to the back-up tong during make up and break out operations. A load cell or torque gauge 15 is located between the torque leg and back-up tong to measure the force therebetween to determine the torque applied by the power tong to the pipe connection between the upper and lower pipe sections 16, 17, respectively. The load cell may be fixed to the back-up tong's arm or to an adjacent surface of the torque leg.
A disadvantage of such prior art arrangements is that a rig operator does not obtain reliable torque measurements during make up operations. Forces exerted by the power tong on the frame structure can flex the torque leg so that it engages the load cell and/or the heel of the back-up tong at an angle rather than in a flat (i.e. parallel) manner, thus providing an uneven force distribution over the load cell and inaccurate torque readings. As a result the rig operator can not be sure whether the pipe connection is being under-torqued or over-torqued. Another problem is that such eccentric loadings on the load cell also lead to premature wear of the cell, which result in further inaccuracies and requires frequent replacement.
What is therefore desired is a novel back-up tong body which overcomes the limitations and disadvantages of the existing arrangements. Preferably, it should provide a means for enhancing the accuracy of torque measurements during make up of pipe connections, and in particular by improving alignment of the load cell with a torque plate or equivalent part of a tong's frame structure during make up operations. It should also reduce wear on the load cell and associated servicing costs.